Method and apparatus for improved determination of range and angle of arrival utilizing a two tone CW radar

Abstract

An improved system is provided for aiming a shotgun-based or other countermeasure system so as to be able to countermeasure incoming rockets or projectiles. In one embodiment a shotgun aimed and controlled by the subject system projects a pattern of pellets to intercept a rocket-propelled grenade or incoming projectile. The fire control system uses a CW two-tone monopulse radar to derive range and angle of arrival within 150 milliseconds, with range and angle of arrival measurements having approximately twice the accuracy of prior CW two-tone monopulse radars. The improvement derives from using all of the information in the returned radar beams and is the result of the recognition that one can use the Sum and Difference signals to assemble a two-by-two Rank One matrix that permits using singular value decomposition techniques to generate range and angle of arrival matrices in which all available information is used and in which noise is eliminated.

Description

FIELD OF THE INVENTION [0001] This invention relates to fire control systems and more particularly to a continuous wave radar for the determination of range and bearing of an incoming missile or projectile. BACKGROUND OF THE INVENTION [0002] Ongoing global conflicts have shown that there is a need for a low-cost solution to the problem of detecting and tracking incoming fire. Currently no system exists to provide troops with an effective automated method for detecting rocket-propelled grenades (RPGs) or other incoming fire. In some cases, for example, helicopter pilots do not even know they are being shot at, either by small arms fire or RPGs. [0003] While passive defenses such as reactive armor or other artifices such as chicken wire have been suggested to countermeasure the RPGs, they are not effective for this purpose. [0004] Moreover, it will be appreciated that the entire point and shoot scenario must be accomplished, for instance, within 150 milliseconds. This is because typic...

AI Technical Summary

Benefits of technology

[0025] It is a finding of the subject invention that all of the information in the CW radar returns can be used to obtain range and angle of arrival. By doing so, one can double the accuracy of the measurement. This doubling is made possible by the recognition that the Sum f1, Sum f2, Diff. f1 and Diff. f2 beams may be assembled into a two-by-two matrix that takes into account all of the information in the Sum and Difference beams. Secondly, this two-by-two matrix has been found to have a special property, namely that it is a Rank One matrix in the absence of noise. Knowing that it is a Rank One matrix, by doing a singular value decomposition on the matrix one obtains improved noise filtering and thus increased accuracy.

[0027] Thus, by doing a singular value decomposition of the Rank One matrix, the result is that angle of arrival accuracies are improved by 100% as are the range estimates, thus to allow the successful pointing and firing of the aforementioned shotgun towards an incoming RPG or other projectile.

[0031] What is therefore accomplished is the ability to provide fire control signals to gimbal a gun, be it a shotgun or other type of countermeasure device, onto a target that is rapidly approaching the two-tone monopulse radar. The entire start-to-finish aim and shoot interval for the subject system is less than 150 milliseconds and provides sufficient pointing accuracy to be able to direct pellets in the direction of the oncoming projectile at precisely the right time to maximize pellet pattern impact. Moreover, the pointing accuracies available with the subject system are sufficiently good so that the pellet cloud will not miss the target.

[0032] Additionally, the velocity information that is obtained as a by-product of the peak detection process is used to inhibit firing if the detected velocities are below, for instance, 100 meters per second. This means that objects such as cars, taxicabs, people and other slow-moving objects will not result in false alarms or misfirings.

[0034] The improvement derives from using all of the information in the returned radar beams and is the result of the recognition that one can use the Sum and Difference signals from both frequencies to assemble a two-by-two Rank One matrix that permits using singular value decomposition techniques to generate range and angle of arrival matrices in which all available information is used and in which noise is eliminated.

[0035] Thus all of the information in the Sum f1, Sum f2, Difference f1, and Difference f2 signals is used by assembling the specialized matrix, with singular value decomposition (SVD) providing cleaned-up information to provide better estimates of range and better estimates of angle of arrival. The recognition that the matrix has a very special structure enables one to both throw away noise contaminating measurements via the SVD process and use all of the information that is available from the radar returns.

Problems solved by technology

Currently no system exists to provide troops with an effective automated method for detecting rocket-propelled grenades (RPGs) or other incoming fire.

While passive defenses such as reactive armor or other artifices such as chicken wire have been suggested to countermeasure the RPGs, they are not effective for this purpose.

Currently available warning suites tend to be too expensive to deploy on every vehicle.

The requirements of such a system are severe in that one must be able to ascertain the trajectory or path of the incoming RPG and to be able to project out a pellet pattern that is sufficiently dense to countermeasure it.

If the shotgun is mispositioned, then the pattern will miss the RPG altogether.

If the pellet cloud intersects the incoming RPG too far away, then the pellets will have dispersed too much to guarantee an RPG kill.

If the range is miscalculated such that the shotgun is fired when the RPG is too close, then the cone is so narrow that any slight aiming error will cause the narrow pellet cloud to miss the RPG.

Note that eight-tenths of a degree accuracy is difficult to attain.

However, upon reflection it will be appreciated that the bloom on the focal plane array of an IR detector is much too large to be able to provide the aiming accuracies required.

However, this approach leads to a very expensive implementation.

Thus, traditional pulse Doppler radars that can measure the required parameters use expensive components and require large spectral bandwidths.

While these systems provide estimates of angle of arrival and range, their accuracy is too poor to support the types of aiming accuracies required to shoot down an RPG or other incoming projectile.

The reason that the prior two-tone monopulse CW radars have been unable to deliver the required accuracies derives from the fact that not all of the information that is developed in the Sum and Difference channels is used.

The sequential processing in essence discards a fair amount of available information from the radar returns and takes up valuable time.

However, this kind of angle of arrival measurement has a number of problems.

First, if there is more than one target in the beam, the system simply does not work.

More importantly, one does not necessarily even know of the existence of multiple targets.

If the true angle is outside the plus or minus four-and-a-half degree limit, one still obtains an answer, but this answer indicates that the angle of arrival is within the four-and-a-half degrees, thus presenting an ambiguity that cannot easily be resolved.

The problem with this approach is that two separate calculations introduce calculation errors.

As will be appreciated, systems that require sequential calculations have inherent accuracy limits.

In the past, there was no convenient way to be able to instantly decide whether to use the Sum f1 and Sum f2 beams or the Diff. f1 and Diff. f2 beams.

This is quite constraining and time consuming because one would have to have first calculated the angle of arrival to make the appropriate estimate.

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